1. Field of the Invention
The present invention pertains to an airway adapter for use with a sidestream gas sampling system, and, in particular, to an airway adapter with a single port that is substantially less sensitive to the ingress of fluids into the sidestream gas sampling system, regardless of the orientation of the port, than conventional airway adapters.
2. Description of the Related Art
Respiratory gas measurement systems are widely used in medical applications and are typically categorized by operating modality into two different types: (1) “diverting” or “sidestream” gas sampling systems; and (2) “non-diverting” or “mainstream” gas sampling systems. A diverting gas measurement system transports a portion of the sampled gases from a sampling site, through a sampling tube, to a gas sensing system. A path (usually optical) is provided through the flow of sample gas to permit the constituents of the gas to be measured by the gas sensing system. Typically, a sample cell is provided at the end of the sampling tube distal from the sampling site. The optical path is provided by means of a window, or pair of windows, disposed on a wall of the sample cell, so that light or other radiant energy passes into to sample cell through the window and travels through the flow of gas in the sample cell to measure the constituents of the gas in the sample cell. The gas sensing system typically includes a radiation emitter, which transmits radiation, such as light, through at least a portion of the gas flow, and a detector, which measures the radiation passing through that portion of the sample gas flow.
A non-diverting gas measurement system does not transport gas away from the sampling site, but measures the gas constituents at the gas sample site, e.g., directly at the patient circuit. This is accomplished by locating the gas sensing system directly on the patient circuit using an appropriate adapter. While mainstream gas measurement systems generally offer better performance because they offer a more direct measurement of the gas constituents, sidestream gas measurement systems have been considered to be better suited for some clinical applications.
For patients receiving ventilatory support, a patient circuit communicates a flow of gas between the ventilator or other pressure support system and the airway of the patient. A patient interface device is provided at the distal end of the patient circuit to communicate the gas from the patient circuit to the patient's airway. A wide variety of different types of patient interface device are known, such as an endotracheal tube, tracheomy tube, or mask. An airway adapter is used to provide access to the respiratory gas of the patient. The airway adapter is disposed in the patient circuit so that gas delivered to and received from the patient passes through the adapter. Gas is drawn from the patient circuit to the sidestream gas sampling system by means of a sampling port provided in the airway adapter. Thus, the airway adapter provides the sampling site from which gas is extracted for measuring its constituents.
With diverting gas measurement systems, a portion of the gas flow is drawn from the patient circuit through at least one port provided in the airway adapter, transported through the sampling tube, and then analyzed by the gas sensing system. In some conventional diverting gas measurement systems, a filter/water trap is located along the gas flow path, typically upstream of the gas sensing system, to prevent water or other contaminants from entering the gas sensing system.
In some conventional airway adapters, the inlet portion of the port from which the gas sample is drawn projects from the wall of the airway adapter. This allows the gas sample to be taken from the central portion of the gas flow in the patient circuit. It is also known to provide the inlet portion of the port in the airway adapter flush with the wall of the adapter. It is further known to provide the outlet portion of the port in a conventional airway adapter such that it projects from the outer wall of the adapter. This projection is used couple the sampling tube to the port. On the other hand, it is also known to provide a recess in the exposed wall of the airway adapter to which the sampling tube connects.
The measurement of the partial pressure of a gas significantly distant from the sampling site requires consideration of several issues: including (1) how to minimize the ingress of contaminants into the sampling tube, (2) what portion of the gas stream is being sampled, and (3) what configuration for the structure used take the gas sample from the gas flow should have.
Airway adapters are typically located as close to the patient's airway as possible in order to minimize the mixing and resulting dampening of the gas waveform that would occur by placing the airway adapter farther downstream from the patient. By placing the airway adapter close to the airway, the humidity of the gas is near 100% RH, and condensation is likely to occur, resulting in water droplets on the interior surface of the airway adapter. Additionally, substances, such as liquid or solid secretions, or mixtures thereof, including mucous, saliva, blood, and condensed water may be present and potentially block or clog the sampling system. The ingress of such substances into the sidestream gas sampling system degrades the performance of the sidestream system and dramatically reduces the lifetime of the sampling tube/filter set.
Because sidestream gas sampling systems divert gas from the gas stream for analysis, it is clinically desirable to minimize the volume of gas that is removed from the gas stream, particularly for patient groups such as infants and neonates, which have relatively small tidal volumes. Therefore, the typical sampling flow rate of 150–180 ml/min is decreased to 50 ml/min for such patients. Additionally, in an intubated patient, airway adapters may be directly connected to the endotracheal tube connector or connected via an elbow or located elsewhere in the breathing circuit. With a direct endotracheal tube connection, a jetting effect may occur with the gas stream being primary centrally located within the airway adapter. With an elbow connection, the gas velocity profile is asymmetric. Therefore, careful consideration of how and where the gas sample is drawn is important to ensure that the sidestream waveform measurement retains as much fidelity as possible.
One approach to reduce the susceptibility to the ingress of liquid is the placement of multiple inlet ports within the gas stream. If one port inlet becomes occluded, the other(s) could continue to draw gas to the sampling system. However, such multiple port designs are complicated and draw air from different portions of the gas stream depending upon the nature and location of the occlusion.
The inlet ports of in conventional sidestream sampling airway adapters also require that the port be placed in a substantially upright position, thereby utilizing gravity to help prevent fluids from entering the sampling system. However, requiring clinical users to maintain the airway adapter in an upright position is frequently unrealistic in today's resource-limited hospital environment. Additionally, the airway adaptor may be incorrectly installed by clinical personnel, and patient movement may cause the airway adapter and its port to rotate, thereby requiring that the adapter be frequently repositioned. Thus, an airway adapter having an inlet port that is insensitive to position so that its orientation need not pay a significant role in the connection of the airway adapter in the patient circuit, would be highly desirable and would reduce the need to reposition the airway adapter.
Therefore, there is a need for an airway adapter with an inlet that is preferably centrally positioned within the airway adapter, relatively insusceptible to the ingress of substances into the sampling system to which the airway adapter is connected, position/orientation independent, and having a design that is inexpensive to manufacture.